(1) Field of the Invention
This invention relates to measurement techniques for determining cavity linewidth of a passive resonator cavity. More particularly, this invention relates to a method for measuring cavity linewidth of a passive resonator which uses a multiple input radiation source to perform such measuring techniques.
(2) Description of Related Art
Heretofore, passive resonator cavities, either linear or ring-configured, have had their cavity linewidth measured according to conventional techniques such as fast finesse and ringdown.
In the fast finesse technique, a series of resonant peaks are displayed on an oscilloscope. The signal shown is detected from the output of a resonator where a coherent radiation source, such as a laser, has injected radiation into the resonator cavity being measured. The output display for fast finesse technique (in the prior art) displayed a series of peaks separated by a distance known as the free spectral range (FSR), as a result of sweeping a piezoelectric transducer (PZT) initially through at least a GHz at a rate of about 30 Hz. This sweeping may be done at the source or the resonant cavity. The free spectral range is the separation in frequency (hertz) between two adjacent longitudinal modes such as Q and Q+1. The free spectral range is usually equal to C divided by L, where C is the speed of light and L is the ring resonator cavity length. Where a linear cavity is being measured, free spectral range is related to C/2L. Using the fast finesse technique, this free spectral range must be measured empirically by reference to the oscilloscope display of the experimental configuration. Additionally, fast finesse technique required a measurement of linewidth measured as FWHM or Full Width at Half-Maximum power. When both of these empirical measurements are made, the finesse of the resonator cavity may be determined by the ratio of free spectral range divided by linewidth. Thus, fast finesse technique required two separate empirical measurements. Both free spectral range and linewidth had to be separately measured in order to provide proper information as to the resonant frequency of the passive cavity. An error in either measurement could introduce error in the derived finesse.
Likewise, the ringdown technique involves the examination of a resonant cavity wherein short pulses are injected into the cavity. The decay time of the short pulses trapped within the cavity is a measure of cavity loss. The output signal on the oscilloscope may be used to make the necessary measurements.